I get error when i use fgets instead of gets - c

Question

This is a picture when I use fgets. (it does not work properly, insanely slow !!)
and This is a picture when it comes to gets. (works fine)
# define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
struct sNode {
struct sNode* tNode;
int G[20];
};
struct tNode {
struct tNode* prev;
struct tNode* next;
int data;
int used;
};
int convert_input(struct sNode** t_ArrayRef, char string[200])
{
int j = 0, i = 0, temp = 0;
int K[20];
while (string[i] != '\0')
{
temp = 0;
while (string[i] != ' ' && string[i] != '\0')
temp = temp * 10 + (string[i++] - '0');
if (string[i] == ' ')
{
i++;
}
(*t_ArrayRef)->G[j++] = temp;
}
return j - 1;
}
int main() {
int i;
char string[200];
char* str[5];
struct sNode* t = (struct sNode*)malloc(sizeof(struct sNode));
str[0] = string;
//fgets(str[0], sizeof(str[0]), stdin); // doesn't works !!!
gets(str[0]); // works !!!
int c = convert_input(&t, str[0]);
int num = t->G[0];
const int a = num;
struct tNode* tNod[6000];
for (i = 0; i<num; i++) {
tNod[i] = (struct tNode*)malloc(sizeof(struct tNode));
}i = 0;
for (i = 1; i<num; i++) {
tNod[i - 1]->data = i;
tNod[i - 1]->used = 0;
if (i != num - 1) {
tNod[i - 1]->next = tNod[i];
}
else {
tNod[i - 1]->next = tNod[i];
tNod[i]->data = i + 1;
tNod[i]->next = tNod[0];
}
}i = 0;
struct tNode* current;
i = 1; int j = 0; int fCount = 0; int zCount = 0;
current = tNod[i - 1];
printf("<");
while (fCount == 0) {
while (current->used == 1) {
current = current->next;
j++;
if (j > num) {
fCount = 1;
break;
}
}
j = 0;
if (i % t->G[1] == 0 && fCount == 0) {
zCount++;
if (zCount != t->G[0]) {
printf("%d, ", current->data, i);
current->used = 1;
}
else {
printf("%d", current->data, i);
current->used = 1;
}
}
i++;
current = current->next;
}
printf(">");
return 0;
}
Could anyone explain to me why I can't get it working using fgets ?

Answer 1

When you use
fgets(str[0], sizeof(str[0]), stdin);
you do not pass the correct size: sizeof(str[0]) is the size of a pointer to char, not the size of the 200-byte char array that you stored in it.
The compiler resolves this sizeof operator at compile time. It has no idea about the value that you put into element zero. In fact, it ignores zero altogether, replacing with sizeof(*str).
Fix this problem by passing the proper size:
fgets(str[0], sizeof(string), stdin);